It's a very good question. Not owning any shades, I have never looked at this and now I see that it is only possible to select a single shade per RM (Rule Machine) trigger or rule. What you will need to do is create a trigger or rule for each shade. Assign the same button (or whatever your trigger is going to be) to each rule. When your trigger activates, both rules will fire at once raising or lowering your shades.
However, I think you will probably want to use a rule, rather than a trigger. A trigger does a single action, so you will only be able to raise or lower the shades with a trigger (one or the other, not both). To do both, you want to create a rule.
Here's Bruce Ravenel's guide to rule machine to help you get started.
